The present invention relates to capacitive pressure transducers. More specifically, the present invention relates to an improved method for forming a seal between a housing and a diaphragm of a capacitive pressure transducer.
FIG. 1A depicts a cross-sectional side view of an assembled prior art capacitive pressure transducer assembly 10. FIG. 1B is an exploded view of the upper housing 40, diaphragm 56 and lower housing 60 of FIG. 1A. Briefly, capacitive pressure transducer assembly 10 includes a body that defines an interior cavity. A relatively thin, flexible diaphragm 56 divides the interior cavity into a first sealed interior chamber 52 and a second sealed interior chamber 54. As will be discussed in greater detail below, diaphragm 56 is mounted so that it flexes, moves, or deforms, in response to pressure differentials in chambers 52 and 54. Transducer assembly 10 provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure between chambers 52 and 54. The parameter provided by transducer assembly 10 indicative of the differential pressure is the electrical capacitance between diaphragm 56 and one or more conductors disposed on an upper housing 40.
Capacitive pressure transducer assembly 10 includes a ceramic upper housing 40, a ceramic diaphragm 56 and a ceramic lower housing 60. The upper housing 40, which generally has a circular shape when viewed from the top, defines an upper face 41, a central lower face 47, an annular shoulder 42 that has a lower face 42a and an annular channel 43 that is located between the central lower face 47 and the annular shoulder 42. Lower face 42a of the annular shoulder 42 is substantially co-planar with central lower face 47. The upper housing further includes a pressure tube 44 that defines a central aperture (or passageway) 48 that extends through the housing 40 from the upper side to the lower side. A metallic conductor 46 is disposed on a center portion of the lower face 47.
The diaphragm 56 is generally a circular thin diaphragm that has an upper face 57 and an opposite, lower, face 59. A metallic conductor 58 is disposed on a center portion of upper face 57 of the diaphragm 56. The diaphragm 56 and the upper housing 40 are arranged so that the conductor 46 of the upper housing 40 is disposed opposite to the conductor 58 of the diaphragm 56. Diaphragm 56 is coupled to the upper housing 40 by an air-tight seal (or joint) 70, which is discussed in more detail below. The seal 70 is located between the lower face 42a of the annular shoulder 42 of the upper housing 40 and a corresponding annular portion of face 57 of diaphragm 56. When sealed, the upper housing 40, seal 70 and diaphragm 56 define a reference chamber 52. Aperture 48 of the pressure tube 44 provides an inlet or entry way into reference chamber 52.
The lower housing 60, which generally has a circular shape, defines a central opening 64 and an upwardly projecting annular shoulder 62 that has an upper face 62a. The upper face 62a of shoulder 62 of the lower housing 60 is coupled to a corresponding portion of lower face 59 of diaphragm 56 by an air-tight seal (or joint) 76. Seal 76 can be deposited and fabricated in a manner similar to that of seal 70. When sealed, the lower housing 60, seal 76 and face 59 of the diaphragm 56 define a process chamber 54.
A pressure tube 66 having an inlet passageway 68 is coupled to the lower housing 60 so that the inlet passageway 68 is aligned with the opening 64 of the lower housing 60. Accordingly, the process chamber 54 is in fluid communication, via opening 64 and inlet passageway 68, with an external environment. In operation, the capacitive pressure transducer assembly 10 measures the pressure of this external environment.
Conductors 46 and 58 of the capacitive pressure transducer assembly 10 form parallel plates of a variable capacitor C. As is well known, C=Aεrε0/d, where C is the capacitance between two parallel plates, A is the common area between the plates, ε0 is the permittivity of a vacuum, εr is the relative permittivity of the material separating the plates (e.g., εr=1 for vacuum), and d is the axial distance between the plates (i.e., the distance between the plates measured along an axis normal to the plates). So, the capacitance provided by capacitor C is a function of the axial distance between conductor 46 and conductor 58. As the diaphragm 56 moves or flexes up and down, in response to changes in the pressure differential between chambers 52 and 54, the capacitance provided by capacitor C also changes. At any instant in time, the capacitance provided by capacitor C is indicative of the instantaneous differential pressure between chambers 52 and 54. Known electrical circuits (e.g., a “tank” circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor C) may be used to measure the capacitance provided by capacitor C and to provide an electrical signal representative of the differential pressure. Conductors 46, 58 can be comprised of a wide variety of conductive materials such as gold or copper, for example, and can be fabricated via known thin and thick film processes or other known fabrication methods. When thin film processes are utilized, conductors 46, 48 may have thicknesses of about 1 μm, for example.
Diaphragm 56 is often made from aluminum oxide. Other ceramic materials, such as glass ceramic monocrystalline oxide materials, however, may also be used. Capacitance sensors having ceramic components are disclosed in U.S. Pat. Nos. 5,920,015 and 6,122,976.
In operation, capacitive pressure transducer assembly 10 is normally used as an absolute pressure transducer. In this form, reference chamber 52 is normally first evacuated by applying a vacuum pump (not shown) to pressure tube 44. After reference chamber 52 has been evacuated, tube 44 is then sealed to maintain the vacuum in chamber 52. A “getter” may also be connected to tube 44 so as to maintain the vacuum in reference chamber 52 over long periods of time. This creates a “reference” pressure in chamber 52. Although a vacuum is a convenient reference pressure, other reference pressures can be used. After the reference pressure has been established in chamber 52, the pressure tube 66 is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling the pressure tube 66 in this fashion delivers the fluid, the pressure of which is to be measured, to process chamber 54 (and to the lower face 59 of the diaphragm 56). The center of diaphragm 56 moves or flexes up or down in response to the differential pressure between chamber 52 and 54 thereby changing the capacitance of capacitor C. Since the instantaneous capacitance of capacitors C is indicative of the position of the diaphragm 56, transducer assembly 10 permits measurement of the pressure in chamber 54 relative to the known pressure in chamber 52.
Transducer assembly 10 can of course also be used as a differential pressure transducer. In this form, pressure tube 44 is connected to a first source of fluid (not shown) and pressure tube 66 is connected to a second source of fluid (not shown). Transducer assembly 10 then permits measurement of the difference between the pressures of the two fluids. Alternatively, reference chamber 52 can be maintained at atmospheric pressure to provide a “gauge” transducer.
As noted above, changes in the differential pressure between chambers 52, 54 cause diaphragm 56 to flex thereby changing the gap between conductor 46 and conductor 58. Measurement of changes in the gap permits measurement of the differential pressure. The gap, however, can also be affected by factors unrelated to pressure. For example, the gap can be affected by changes in temperature. Since the components of transducer assembly 10 can be made from a variety of different materials, each of which has its own characteristic coefficient of thermal expansion, temperature changes in the ambient environment can cause the diaphragm 56 to move closer to, or further away from, conductor 46. Fortunately, changes in the gap caused by temperature changes are characteristically different than changes in the gap caused by changes in differential pressure. To compensate for changes in the gap that are caused due to changes in the ambient temperature, it is known to include a second conductor (not shown) that is disposed adjacent to conductor 46 on the lower face 47 of the upper housing 40. In such an embodiment, conductors 46 and 58 form parallel plates of a variable capacitor C1 and conductor 58 and the second conductor form parallel plates of a variable capacitor C2. The two capacitors, C1 and C2, may be used by known methods to reduce the transducer's sensitivity to temperature changes.
The upper housing 40 is positioned so that the lower face 47, and any conductors disposed thereon, are disposed in a plane that is parallel to the plane defined by the conductor 58 (i.e., diaphragm 56) when the pressures in chambers 52, 54 are equal. As discussed above, the capacitance defined by the conductors 46, 58 depends upon the gap (i.e., axial distance) that exists between these opposing conductors. The gap, which is relatively small (e.g., on the order of 0.0004 inches (10–12 μm)), depends, in part, upon the thickness of the seal 70 and the shape and configuration of the upper housing 40 (e.g., the amount that lower face 42a is out of plane, i.e. offset, with lower face 47, if any).
A method for forming seals 70 and 76 is disclosed in U.S. Pat. No. 6,122,976. In that method, a seal is formed by placing solid glass beads between two surfaces, applying a compression force between the two surfaces and then melting the sealing beads. Upon melting, the melted beads flow into the space between the two surfaces. Upon cooling, the flowed seal bead material forms a seal between the two surfaces.
FIG. 2 shows a bottom view of the upper housing 40 of FIGS. 1A and 1B. In accordance with the teachings of U.S. Pat. No. 6,122,976, glass particles are mixed with a binding agent(s) and a solvent(s) to form a paste material. The paste material is then deposited as a pattern of sealing beads 72 on a surface where the seal is to be formed, e.g., lower face 42a of shoulder 42 of the upper housing 40 and upper face 62a of shoulder 62 of the lower housing 60. The pattern of sealing beads 72 can be deposited and formed on the surface by utilizing suitable screen-printing or pad/brush printing deposition processes. As discussed in more detail below, the sealing beads 72 are deposited so that open channels 78 exist between the sealing beads 72. After the pattern of sealing bead (paste) 72 has been deposited on the surface, the sealing beads 72 are subjected to a drying process, a “burn-off” process and then a prefusion/sintering process. In each subsequent step, the sealing beads 72 are exposed to increasingly higher temperatures. For example, the sealing beads 72 may be heated to 100–150 degrees C. (Celsius) during the drying process, heated to 325–375 degrees C. during the burn-off process and heated to 490–500 degrees C. during the prefusion/sintering process. The deposited sealing beads (paste) 72 are hardened in the drying process so that they can resist handling. During the burn-off process, some of the solvents and binding agents are burned-out of the paste. If the burn-off process is not performed adequately, the seal may not be impermeable and also may be structurally inadequate. When the sealing beads 72 have been sufficiently degassed (burned-off), the temperature is further increased to perform the prefusion/sintering step. During the prefusion/sintering process, the glass particles that are present in a bead 72 fuse together. The beads 72, however, do not flow into the open channels 78 during the prefusion/sintering step. After the prefusion/sintering step, the sealing beads 72 are then allowed to cool. After cooling, the sealing beads 72 can then be mechanically worked, e.g. polished, so that the sealing beads have a desired height. In some applications, the desired height of the (unmelted) sealing beads 72 is established at about 20–24 μm, for example.
The pattern in which the beads 72 are deposited (shown in FIG. 2) affects the ability of the beads 72 to fully degas while the seal 70 (or seal 76) is being formed. To facilitate the degassing of the sealing beads 72, it can be advantageous to deposit the sealing beads 72 with channels 78 between the sealing beads 72. The dimensions of the cross-sections of the sealing beads 72 and the channels 78 that are disposed between them are chosen so that the desired degassing effect can be achieved. In one exemplary embodiment, the sealing beads 72 have a diameter of between 0.1–0.5 mm and the channels 78 have widths of about the same magnitude.
After the sealing beads 72 have been deposited and prepared on the lower face 42a and upper face 62a in the manner described above, the diaphragm 56 is aligned with the upper housing 40 so that the sealing beads 72 located on the lower face 42a come into contact with the sealing area of the upper face 57 of the diaphragm 56 and the lower housing 60 is aligned with the diaphragm 56 so that the sealing beads 72 located on the upper face 62a come into contact with the sealing area of the lower face 59 of the diaphragm 56. A compression force is then applied to the upper housing 40, diaphragm 56 and lower housing 60 in a direction that is generally perpendicular to the orientation of the diaphragm 56. A higher temperature (i.e., higher than that which was applied during the prefusion/sintering step) is then applied to melt the sealing beads 72. Upon melting, the sealing beads 72 flow to fill the voids (i.e., channels 78) that exist between the shoulder 42 of the upper housing 40 and the upper sealing area of the diaphragm 56 and between the shoulder 62 of the lower housing 60 and the lower sealing area of the diaphragm 56. Upon cooling, the sealing beads 72 thus form the air-tight seals 70, 76 which are located between the diaphragm 56 and the upper housing 40 and lower housing 60, respectively. To form a seal 70 (and seal 76) having a desired height (i.e., thickness) and area, the cross-sectional areas and heights of the unmelted sealing beads 72 is set so that the total volume of the sealing bead 72 material is sufficient to form the desired seal 70, i.e., the total volume of the sealing beads 72 is generally equal to the volume of the desired seal 70.
The performance characteristics of a capacitive pressure transducer can be adversely affected if the conductors of the capacitive pressure transducer cannot be accurately located and oriented relative to each other. For example, if the gap between opposing conductors 46, 58 is not established in a controlled manner with tight dimensional tolerances, the capacitive pressure transducer may have unacceptable performance characteristics. Further, if the gap can not be consistently controlled, it may be difficult to produce large numbers of transducers that all have the same performance characteristics.
The sealing method described above does not necessarily insure that the formed seal 70 has an accurate and constant thickness. For example, the seal 70 may be too thick or too thin if an excessive or insufficient amount of sealing bead 72 material is used to form the seal 70. Also, the thickness of the seal 70 may not be constant if the compression force that is applied between the upper housing 40 and the diaphragm 56 during the melting and cooling steps is not uniform.
A need therefore exists for a method of accurately forming a seal between a housing and a diaphragm of a capacitive pressure transducer.